Basjan

You build a solar heater with 40mm class 3 PVC pipe cut to e.g. 1m and plastic U-joints joining them. Inlet accepting coldwater and outlet feeding the inlet of your geyser you preheat the water before it inter the geyser reducing the enercy required to heat the water, like solar panels for a swimming pool.

Another option is to run water through 40 mm pipe with U - connector and then supply geyser with warm water and use a small 650 w or 1kw element to topup to 60 degrees. Just require proper insulation on geyser